1. Field of the Invention
The invention relates generally to a capacitive micro-electro-mechanical switch and method of manufacturing the same. More particularly, the invention relates to a capacitive micro-electro-mechanical switch for use in the radio frequency (RF) and the microwave that are driven by an electrostatic force and method of manufacturing the same, capable of simultaneously improving insertion loss and isolation characteristics.
2. Description of the Prior Art
Generally, an electron system used in the radio frequency and the microwave band includes a semiconductor switch such as a field effect transistor (FET) or a p-i-n diode in order to control the signal. However, this type of the semiconductor switch has problems like high insertion loss, low isolation, signal distortion, and the like. In order to solve these problems of the semiconductor switch, a research on a micro-electro-mechanical switch has recently been widely made.
The micro-electro-mechanical switch is driven by the electrostatic force. The micro-electro-mechanical switch is classified into a resistive type and a capacitive type depending on its driving mode.
The resistive switch is formed on two signal transmission lines spaced apart with a given air gap. The resistive switch includes a deflecting plate used as the top electrode and a ground line used as the bottom electrode. In the above, the deflecting plate is bent downwardly by the electrostatic force and is also electrically connected to the signal transmission lines. Through this structure, an ON/OFF operation of the resistive switch is completed. In other words, if the switch is in the OFF state since the electrostatic force is not applied, the signal is isolated. On the other hand, if the switch is in the ON state since the electrostatic force is applied, the signal is transferred.
On the other hand, the capacitive switch includes a deflecting plate of the top electrode connected to the ground line, a signal transmission line of the bottom electrode, and a dielectric film formed on the signal transmission line at a region where the deflecting plate and, the signal transmission line are intersected. In the above, if the deflecting plate is bent toward the signal transmission line by means of the electrostatic force and then contacts the dielectric film formed on the signal transmission line, the signal is bypassed from the signal transmission line to the ground line through the deflecting plate, by means of capacitance of the capacitor having a structure of the deflecting plate/dielectric film/signal transmission line. In other word, if the switch is in the OFF state since the electrostatic force is not generated, the signal is transferred. On the other hand, if the switch is in the ON state since the electrostatic force is generated, the signal is isolated.
As above, the resistive switch and the capacitive switch have not only different structure and operation but also a different frequency region. In detail, the resistive switch is mainly used at a low frequency region (for example, 0 Hz˜20 GHz) and the capacitive switch is mainly used at a high frequency range (for example, 20 GHz˜60 GHz).
The structure and operation of the conventional capacitive micro-electro-mechanical switch will be described by reference to the drawings.
FIG. 1 is a perspective view of one conventional capacitive micro-electro-mechanical switch for explaining a structure of the switch.
Referring now to FIG. 1, the capacitive micro-electro-mechanical switch includes a signal transmission line 110, a deflecting plate 120, a dielectric film 130, ground lines 140 and a metal post 150.
In the above, the signal transmission line 110 serves as the bottom electrode and the deflecting plate 120 serves as the top electrode. The dielectric film 130 is formed on the signal transmission line 110 at a region where the signal transmission line 110 and the deflecting plate 120 are intersected. The ground lines 140 are formed on the substrate with the signal transmission line 110 intervened between them. The metal posts 150 are formed on the ground lines 140 at a region where the deflecting plate 120 and the ground lines 140 are intersected. The deflecting plate 120 is fixed to the metal posts 150 and is suspended at a given space above the dielectric film 130. Meanwhile, the deflecting plate 120 is fixed to the metal posts 150 in a direction vertical to the signal transmission line 110.
FIG. 2A and FIG. 2B are conceptual drawings for explaining the operation of the capacitive micro-electro-mechanical switch shown in FIG. 1.
Referring FIG. 2A, if the switch is in the OFF state, a voltage is not applied between the signal transmission line 110 and the ground lines 140. Thus, the deflecting plate 120 is fixed to the ground lines 140 by the metal posts 150 and is floated over the dielectric film 130. Therefore, the signal is normally transferred through the signal transmission line 110.
By reference to FIG. 2B, on the contrary, if the switch is the ON state, a voltage from a voltage supply means 160 is applied between the signal transmission line 110 and the metal post 150. The deflecting plate 120 is thus bent toward the dielectric film 130 by means of the electrostatic force and then contacts the dielectric film 130. Therefore, a capacitor C100 having a structure in which the signal transmission line 110, the dielectric film 130 and the deflecting plate 120 are sequentially stacked is formed. The signal is bypassed from the signal transmission line 110 to the ground lines 140 through the deflecting plate 120 by means of capacitance of the capacitor C100.
At this time, as the surfaces of the signal transmission line 110 and the dielectric film 130 formed on it are not perfectly smooth, the deflecting plate 120 does not completely contact the dielectric film 130 but an air gap 170 is generated between the deflecting plate 120 and the dielectric film 130. As a result, the air gap 170 causes to degrade an electrical characteristic of the switch since it serves to reduce capacitance of the switch in the ON state.
FIG. 3A and FIG. 3B are conceptual drawings for explaining a structure and the operation of another conventional capacitive micro-electro-mechanical switch.
The capacitive micro-electro-mechanical switch shown in FIG. 3A and FIG. 3B is different from the structure of FIG. 1 in that it further includes an assistant electrode 121 formed on the dielectric film 130. As such, if the assistant electrode 121 is further included on the dielectric film 130, it can prevent reduction in capacitance in the ON state even though the deflecting plate 120 contacts only some regions of the assistant electrode 121 formed on the dielectric film 130. Therefore, even though an air gap 170 is generated between the deflecting plate 120 and the dielectric film 130, it can prevent degradation in the electrical characteristic of the switch due to the air gap 170.
Capacitance (Coff) when the capacitive micro-electro-mechanical switch in the OFF state, is equal to the sum in which capacitance (Cair) of the air gap and capacitance (Cdielectric) of the dielectric film 130 are serially connected, as in the mathematical equation 1 below. At this time, as Cair is relatively very low compared to Cdielectric, Coff has a value approximate to Cair.Coff=Cair*Cdielectric/(Cair+Cdielectric)˜Cair=∈airA/hair  [Equation 1]
On the contrary, capacitance (Con) in the ON state is equal to Cdielectric as in the mathematical equation 2 below.Con=Cdielectric=∈dielectricA/hdielectric  [Equation 2]
In Equations 1 and 2, hdielectric indicates the thickness of the dielectric film 130, hair indicates the thickness of the air gap between the dielectric film 130 and the deflecting plate 120, ∈dielectric indicates the dielectric constant of the dielectric film 130, ∈air indicates the dielectric constant of air, and A indicate an area of a region where the deflecting plate 120 and the signal transmission line 110 are overlapped.
The capacitive switch having a good electrical characteristic has a very low OFF capacitance and thus has a low insertion loss since most of signals are transferred along the signal transmission line. Also, the capacitive switch having a good electrical characteristic has a very high ON capacitance and thus has a good isolation characteristic since most of the signals are bypassed to the ground lines. As a result, the good capacitive switch must have a very high ON/OFF ratio (Con/Coff) of capacitance.
At this time, a method of making low the OFF capacitance (Coff) includes a method of reducing an area (A) of a region where the deflecting plate and the signal transmission line are overlapped, and a method of increasing the thickness (hair) of the air gap. If the former method is used, however, there is a problem that the ON capacitance (Con) becomes also small. If the latter method is used, there is a problem that the driving voltage of the switch in proportion to hair3/2 must be high.
Meanwhile, a method of making high the ON capacitance (Con) includes a method of increasing the area (A) of the region where the deflecting plate and the signal transmission line are overlapped, a method of reducing the thickness(hdielectric) of the dielectric film, and a method of increasing the dielectric constant (∈dielectric) of the dielectric film.
The first method can increase Con. However, this method has problems that Coff is increased and the size of the switch is also increased, as described above. Next, in the second method, if the thickness of the dielectric film is reduced below a given value, the dielectric breakdown is caused. Due to this, the second method has a limitation in reducing the thickness of the dielectric film. The third method is the most effective method to increase the ON capacitance. In this method, STO (strontium titanate oxide; ∈=30˜120) or BSTO (barium strontium titanate oxide; ∈>200) having a high dielectric constant is used as the dielectric film instead of silicon nitride (∈=6˜8) commonly used.
In the above conventional capacitive micro-electro-mechanical switch, a flat type capacitor of a two-dimensional structure is used. Thus, an area of the capacitor having the stack structure in which the deflecting plate, the air layer and the signal transmission line are stacked in the OFF state and an area of the capacitor having the structure in which the deflecting plate, the dielectric film and the signal transmission line are stacked in the ON state are equal. Due to this, if the area of the capacitor is increased, the isolation characteristic is improved since the ON capacitance (Con) is increased. However, there is a problem that the insertion loss is also increased since the OFF capacitance (Coff) is increased. Therefore, the conventional capacitive micro-electro-mechanical switch has a structural problem in increasing the ON/OFF ratio (Con/Coff) of capacitance.